Alarm system



A. M. CURTIS ALARM SYSTEM June 14; 1927. I

Filed Aug. 16. 1922 lnvenfol": n M. Cuff/'5 Affy.

Patented June 14, 192 7. i

UNITED STATES PATENT. OFFICE.

nusrnn 1a. scans, on EAST olmnen, new JERSEY, Assmncn TO wns'rnnn momnrccomrnmr, mconronarnn, or raw YORK, .N. Y., A CORPORATION 01* NEW YORK.

ALARM SYSTEM.

Application filed August 1c, 1929.. Serial No; 5s2,i10.

This invention relates to alarm. systems and particularly to burglar alarm systems.

It is an object of. the invention to provi'de a newand improved alarm system. operating upon alternating current and in which it is impossible for an unauthorized person to substitute a resistance for the subscribers alarm network and. work unmolested for a considerable time.

Another object of the invention is to provide an alternating current. alarm system in which the alarm is made to depend not only on the strength of the current in the line, but also onitsphase relation to the supply voltage. a

This alarm circuit is normally closed through a relatively low frequency A. C. generator atthe central station through an A, G. galvanometer to the line circuit and thence through the protective network at the subscribers station. An. alarm device is connected across the line at the subscribers station in such a manner as to be inoperable by'the normally .low frequency alternating current, but is designed to operate on a signaling current of an easily audible: fr.e.-'

the following description taken. in. conjunc.

tion with the. accompanying drawing which shows a diagrammatic vrew of a circuit embodying the invention.

Referring to the drawing, the invention in U its preferred form consists of a central or control station A, and. a localIor. subscribers stat-ionB, interconnectedby means of asingle li'newire 1 which, if desired, may be replaced by a pairofwires and the ground connection eliminated. At the central'stationA there isprovided a galvanometer 2,

tively high frequency to the line.

or any other visible or audible alarm isalso provided at the central station to be operated from the galvanometer contacts 6. A. second generator 10 is also located at the central station and is adapted to be connected to the line circuit upon the depression of the key 8,-th1s generator providing current of'rela A network of wires 11 is provided at the subscribers premises in the; usual manner and is so disposed as to cover all probable points of ingressand so arranged that'breaking or groundinga wire or crossing. two

wires of. the network will cause an alarm. Atchoke-coil '12 ofproper impedance is con nected in the networkci'rcuit in such a way thatithejoining of any two wiresof the; nets Work will short-circuit the coil. A howler 13, which may be any type ofelectrically, operated telephone receiver or an electrical- 1y vibrated reed device, is connected across the incoming line through the condenser 14, which is of value necessar toprevent any noticeable noise being pro need by the low frequency alternating currentfrom the gen:-

erator 7, which is used to operate the. alarm 9,. but, passes the higher. frequency signaling current of the generator 10. A signaling key 15 is also provided at the subscribers station. and is adapted to ground the incoming line when depressed, thus operatingthe galvanometer and hence the alarm signal 9 at the central station, allowing the, subscriber at station B. to signalthe iicfln-tral station. The galvano neter' 2 isso'designed thatthe motion of its pointer 5 is. practically dead heat] An adjustable spring 16 is: attached to the moving coil 4 in order to compensate for the deflection ofthe, galvanometer coil (due to: the'normal current through the line.

When conditions are normal, the alarm circuit maybetraced from the main. generator 7, through moving coil 4 of the galvanometer 2, conductor 17, key 8, line wire i, one side of the network 11, choke coil 12, through the other side 01" network 11 to ground. The howler circuit at the station B has no appreciable eliect on the operation of the alarm circuit as its impedance to the normal alarm frequency is very high. The field circuit of the galvanometer is bridged across the alarm frequency generator 7 As the effect of this circuit is negligible, the current in the line lags behind the voltage by an amount controlled by the inductance of the choke coil 12. On this account the current in the fixed coil 3, which is bridged across the generator 7 and the moving coil 1 are not inphase, and the deflection of the moving coil depends on both the strength and the phase of the current flowing through it. As pointed out above, the normal defiec tion of the galvanometer may be compensated and the galvanometer pointer brought to the midpoint of its scale by means of the spring 16. It is then evident that any change in either strength or phase of the line current will give a deflection of the pointer from it normal position. It a burglar enters the subscribers premises, by breaking one of the network wires 11 the galvanometer pointer will be deflected toward the open side contact by the spring, operating the alarm signal. If he short circuits the network the pointer will deflect to the ground side contact and also operate the alarm.

In this system it is impossible to avoid sounding an alarm by substituting a false network for the subscribers network since all subscriberslines and networks are different, and as various size choke coils are used to terminate each network, the current strength and phase angles in various lines will vary over a large range, probably from to m. a. and from 30 to degrees. It a burglar should attempt to enter the protectedpremises without operating the alarm and it should be possible for him to work on the leads from the central station without being interfered with, he might determine the current in the line by inserting a milliammeter without opening the circuit. He then might gradually replace the subscribers network by a resistance box in such a way that the line current would not be appreciablychanged. It will be evident, however, that such a substitution would send an alarm due to the change in phase angle of the line current caused by shunting'out the choke coil 12 in the subscribers network. Since the alarm is operated by a change either in current or phase, the substitution of a false network for the subscribers network would involve the measurement of the line current and phase angle, and there is no A. C. wattmeter in existence which will give an indication of the phase of such small currents and voltages as are commonly used on such an alarm line without sending an alarm.

The operation of this system by the subscriber and central oflice operator for the purpose of signaling each other is as follows: Upon closing his premises at night the subscriber may signal the central oflice operator by pressing and releasing his key 15, which operates the alarm 9 over a circuit extending from ground, through key 15, line wire 1, key 8, conductor 17, galvanometer coil '4, generator 7 to ground. The deflection ot' galvanometer coil 4 operates alarm 9 over a circuit extending from generator 7 through coil 4, pointer 5, contact 6, alarm 9, to ground. The central ofiice operator could then note that the galvanometer pointer stood at the proper point and signal the subscriber by pressing the key 8 and causing the howler 13 to ive an audible tone, the operating circuit For the howler extending from generator 10, through key 8, line wire 1,'condenser l4, bowler 13 to ground. When the subscriber enters his premises in the morning an alarm is sounded. He may, however, immediately signal the central office operator by ressing the key 15 and sending a suitable co e signal which the operator may read on the galvanometer.

When an all-metallic circuit is used in this system the lines may be phantomed and by using more than one alarm and signal frequency, a multiplex s stem may be operated. This and other 'epartures from the details herein shown and described will be obvious to those skilled in the art and accordingly it is not intended to limit the invention except as defined in the appended claims.

lVhat is claimed is 1. -An alarm system comprising a line circuit interconnecting a central station and a substation, an alarm device and a source of alternating current at said central station energizing said line circuit, a galvanometer at said central station, said alvanometer having its hold coil excited ocally by said source of current and its'moving coil connected in series with said line circuit, and a spring for normally holding the mov ablecoil against the torque exerted by the magnetic field in each of said coils, said movable coil being responsive to a change in the relation of the phase angle of the currents in the field and movable coils for operating said alarm device.

2. An alarm system comprising a line circuit interconnecting a central station and a substation, an alarm device, a source of alternating current at said central station, a choke coil atsaid substation connected to said line'circuit, and a galvanometer at said central stationhaving its field coil locally excited by said source of alternating current and having its moving coil connected in series with said line circuit, and means for balancing said movable coil against the torque exerted by the currents in said coils whereby the moving coil is rendered normally ineffective with respect to said device but responsive to changes in the reactance of the choke coil and a consequent change in the phase angle of the currents in the field coil and the movable coil for causing the operation of said alarm device.

In Witness whereof, I hereunto subscribe my name this 10th day of August A. D.,

AUSTEN M. CURTIS; 

